1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method, and specifically to reducing color unevenness which occurs when printing is performed by a bidirectional reciprocating scanning operation.
2. Description of the Related Art
Recently, office automation equipments such as personal computers or word processors come into wide use, and various printing apparatuses such as printers for outputting information prepared by the OA equipments are provided. Among the printing apparatuses, a serial type ink jet printing apparatus that executes scanning (scan) of an ink jet print head so as to perform printing is widely used as a printing apparatus capable of perform printing of a high quality and a high speed with relatively low cost.
In such a serial type printing apparatus, bidirectional printing is known as one of high-speed printing techniques. In the bidirectional printing, printing is performed by a forward scanning operation and a backward scanning operation of the print head. The bidirectional printing is performed by a method of completing the printing by a one-time reciprocating scanning operation or a method of completing the printing by a plural-time reciprocating scanning operation. The latter method is called a multi-pass printing method.
In the multi-pass printing method, a unit area for which printing is completed by plural-times of scan is printed with different nozzles in each of the plural-times of scan. More specifically, the nozzles of a print head is divided into a plurality of groups and a printing medium is conveyed by an amount corresponding to one group in each scan so that the plural-times of scan with which different nozzle groups are sequentially associated are performed. According to this printing method, unevenness in an ejection amount or an ejection direction of each nozzle can be dispersed into the plural-times scan and thus density unevenness due to the unevenness in the ejection amount or the ejection direction becomes not conspicuous.
As described above, according to the bidirectional printing, it is possible to obtain a printing speed which is twice as high as that of unidirectional printing for performing printing with a scanning operation of one-direction. In addition, it is possible to perform high-speed and high-quality printing by performing the multi-pass printing together.
However, in the bidirectional printing, it is also known that color unevenness or density unevenness due to an ejection order of respective color inks may occur.
FIGS. 1A to 1C are views illustrating the color unevenness due to the bidirectional printing, which shows a state in which cyan ink and magenta ink ejected from a print head respectively lands on a printing medium. A dot 3501 shown in FIG. 1A is a dot which is formed on the printing medium with the magenta ink ejected previously in a forward scan and a dot 3502 shown in FIG. 1B is a dot which is formed on the printing medium with the cyan ink ejected after the magenta ink in the same forward scan. That is, the print head or ejection opening (nozzle) arrays for ejecting the magenta ink and the print head or ejection opening arrays for ejecting the cyan ink are arranged in this order in the forward direction. In this case, in the forward scan, the magenta ink is first ejected and the cyan ink is then ejected. The respective dots formed with these inks have the same area (fixing area) when these dots do not overlap with each other, like the dots 3501 and 3502. However, in a case where a blue dot is printed, these inks are ejected so as to overlap with each other and thus the fixing area of the formed dot may be different from that of a single dot.
FIG. 1C is a view showing a state in which the magenta ink is first ejected and the cyan ink is then ejected so as to print the dot while performing scanning of the print head in the forward direction which is a +X direction. In FIG. 1C, a reference numeral 3503 denotes a blue dot (main dot) in which the magenta ink which is previously ejected and the cyan ink which is subsequently ejected overlap with each other. In contrast, a reference numeral 3504 denotes a cyan portion formed in the periphery of the main dot by overlapping the subsequent cyan ink with the previous magenta ink. As described above, when the magenta and cyan inks are ejected in this order, a blue dot is formed by the magenta ink which is previously ejected and the cyan ink which is subsequently ejected and the portion formed by the subsequently ejected cyan ink alone is formed in the periphery of the blue dot. This phenomenon notably occurs when printing is performed using a dye ink with respect to a paper (a coated paper) in which a coating layer including a metal oxide such as alumina or silica is formed on a base material. More specifically, the magenta ink which is previously ejected first lands on the printing medium and is permeated into and fixed on a surface layer (the coating layer) of the printing medium. Meanwhile, a part of the cyan ink which is subsequently ejected is permeated into and fixed to the portion, to which the magenta ink is fixed, so as to form the blue dot and the remaining part of the cyan ink which cannot be permeated into the portion to which the magenta ink is fixed is permeated into and fixed to the periphery of the blue dot. In this way, when the dot is formed by ejecting inks of different colors so as to overlap with each other, the fixing area is spread and thus a difference in color occurs due to the overlapping order. In this example, when the magenta ink and the cyan ink are sequentially ejected in the +X-direction scanning, a blue color with a cyan tint is obtained. In contrast, when the cyan ink and the magenta ink are sequentially ejected in the −X-direction scanning, a blue color with a magenta tint is obtained.
As described above, when the bidirectional printing is performed, the difference in the color may occur due to inversion of the ejection order of the inks between the forward scan and the backward scan and thus color unevenness may occur in the whole printed image. For example, in the method of completing printing by each one-time of forward and backward scans, respective colors of whole areas completed by respective forward and backward scans are different from each other and thus the different colors alternately appear from scan area to scan area. Even in the multi-pass printing, density unevenness or color unevenness, in which the color alternately varies according to the unit areas, occurs depending on the number of times of scanning for completing printing of the unit area.
So called a symmetrical head is known as a configuration for solving the color unevenness due to the ink overlapping order of the reciprocating scanning operation (Japanese Patent Laid-Open No. 2001-171151 and Japanese Patent Laid-Open No. 2005-001336). In these documents, two ejection opening arrays or print heads for cyan and magenta colors for example are provided and these arrays or heads for each color are symmetrically arranged in a main scan direction. Then, the different ejection opening arrays or print heads of each of cyan and magenta are used in the forward and backward directions so that the overlapping orders of cyan and magenta inks in the forward direction and the backward direction become equal to each other.
However, in the above prior arts, since at least the two ejection opening arrays or print heads of symmetrical arrangement are provided for one color, there may be problems that the size of the print head is increased and thus the size of the apparatus and cost are increased. That is, the configuration of the symmetrical head can solve the difference in the overlapping order of the reciprocating scanning, but has an inherent problem that additional ejection opening arrays or print heads are required.
The present invention can reduce color unevenness due to the difference in the overlapping order without employing the additional configuration. More specifically, the present invention is made by focusing on a portion of different color generated according to a difference in the overlapping order of different color inks in a dot formed by overlapping the different color inks, to decrease or substantially remove a color difference between the portions of different colors and then reduces color unevenness of a whole image.
As described above referring to FIG. 1, when inks are ejected in an overlapping manner, the dot of the ink which is subsequently ejected is spread to be formed larger. The spread portion has a different color due to the difference in the overlapping order in the reciprocating scanning. In the example shown in FIG. 1, a portion of the dot 3504 excluding the dot 3503 denotes the portion having the different color due to the difference in the overlapping order in the reciprocating scanning.